Friday, March 27, 2015

A North Atlantic nasty surprise: AMOC slowing faster than expected

Atlantic Meridional Overturning Circulation slowing much faster than expected


It seems that the 2004 Hollywood blockbuster disaster movie The Day after Tommorrow, while highly exaggerated, had the seed of scientific fact based upon the possible disruption that could occur from the slowdown or catastrophic cessation of the Atlantic ocean Meridional Overturning Circulation (AMOC), also known as the thermohaline circulation or Great ocean conveyor belt.

A new study by Rahmstorf et al (2015) published in Nature Climate Change has found evidence for a slowdown of the overturning circulation. This is based upon multiple lines of observation suggesting that over recent decades the current system has been weaker than ever before in the last century, or even in the last millennium.

The Gulf Stream is part of this circulation system with warm waters, which are naturally lighter, flowing north, and responsible for the mild climate of north western Europe. In the sub-polar region these waters cool, become more dense, and sink to the bottom before they start flowing south along the ocean bottom towards the tropics, then the South Atlantic and the Southern Ocean. The conveyor belt of currents connects the Atlantic, Indian, Pacific and Southern Oceans providing heat transfer and mixing throughout the globe.

Related Links: What’s going on in the North Atlantic? at Realclimate Blog | New Research Shows Exceptional Slowdown In Major Atlantic Ocean Currents Greg Laden's Science Blog


With warming and melting of the Greenland Ice sheet we are seeing a substantial freshening of the water south east of Greernland. “Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation,” says Jason Box of the Geological Survey of Denmark and Greenland.


The freshwater from the ice sheet is diluting the ocean water. As freshwater is less saline, and it is also less dense and has therefore less tendency to sink into the deep.

“So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further,” explains Box.




You can see on global temperature anomaly map just how much different the cool temperature anomaly of the waters south east of Greenland is compared to the warming anomaly of the rest of the globe.

“It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up,” says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, lead author of the study that was published in Nature Climate Change. Previous research had already indicated that a slowdown of the so-called Atlantic meridional overturning circulation might be to blame for this.

“Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” says Rahmstorf.

The climate models have also underestimated the ice sheet melt water impact to the observed cooling.

“Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both,” said Michael Mann of Pennsylvania State University in the US, another author of the paper. “That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”

Coral proxy data provided by Mann enabled the research to stretch back the time series for changes in the AMOC back to 900AD. This shows that the circulation has been relatively stable, although with some variability for the last 1000 years. The AMOC index for 1975 to 1995 shows a striking fall with the chance that this was natural variability found to be < 0.005 using the Monte Carlo method to estimate the odds. The cooling impact in the North Atlantic may moderate the temperature rise with global warming, but certainly won't produce the extreme megastorms and abrupt climate change as depicted in the The Day after Tomorrow 2004 film.

Ten years ago, around 2003 and 2004, the risk of failure of the thermochaline circulation was first widely raised.

Indeed, I helped write an editorial highlighting the growing problem of climate change for the global Indymedia site using a Thom Hartmann's article - How Global Warming May Cause the Next Ice Age - as the lead-in. There were other articles around at the time, including one by Geology Professor Bill Maguire and by NASA. The NASA article warned that "Global warming could plunge North America and Western Europe into a deep freeze, possibly within only a few decades."



For fifty or so years the data on the Atlantic Meridional Overturning Circulation was collected by surveys by ship about once every decade. The data simply wasn't consistent enough to provide detailed information on the changes to the current. In the early years of this century a major program of the British National Oceanography Centre (NOC) of placement of buoys to monitor the AMOC at 26.5N degrees across the Atlantic between the Canary Islands and the Bahamas. These buoys have provided temperature, salinity and pressure measurements of the ocean twice daily for the last decade.

Results highlighted in a media release by the National Oceanography Centre explain:
Firstly, it had been thought that the strength overturning of the AMOC would weaken due to climate change. However, the ocean sensors have detected that the AMOC is now declining faster than anticipated, which could potentially have a long-term impact on Britain's climate.

Secondly, the results revealed that the AMOC was significantly more variable than had been previously thought. Thirdly, the data also appeared to confirm that the AMOC had a direct impact on Britain's winter weather, which could be specifically seen with respect to the harsh winter of 2010/11. The measurements showed that the strength of the AMOC in 2009/10 was much lower, which affected sea surface and atmospheric temperature -- and seemed to directly affect Britain's weather months later. The slowdown in the AMOC in 2009/10 also raised sea levels in New York by 13cm -- 4 times the global average sea level rise.


So what are the main impacts of the overturning circulation slowing down?

“If the slowdown of the Atlantic overturning continues, the impacts might be substantial,” says Rahmstorf. “Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”

Temperature rise moderated


While in The day before Tomorrow Europe and North America were plunged into a new ice age in a period of a week or two. During 2003 and 2004 northern hemisphere cooling was seriously debated as a possibility over decadal time frames by some climate scientists. This is now seen widely as most unlikely to happen. More realistically, as the Gulf stream slows down rising temperatures for the east coast of the United States and Northern Europe may be moderated somewhat by some cooling preventing larger increases in temperature.

Enhancing extreme sea level rise


Another large impact of a slowing current is an increase in extreme sea level rise. The mean global sea level rise for the last 20 years has been about 3 mm per year, mainly from thermal expansion but with some contribution from mountain glaciers and ice sheet mass loss.

But regional factors are also important and are on top of this global mean rise. Because of the coriolis force pushing water to the east in the northern hemisphere and the ekmen flow pulling water from the coast, a measured 30 per cent reduction in the Atlantic meridional overturning circulation (AMOC) during 2009-2010 caused an extreme sea-level rise event. The coastal sea level north of New York City to Newfoundland jumped by up to 128 mm in this two year period. This is an unprecedented event in the tide gauge record, with statistical analysis suggesting it was perhaps a 1-in-850 year event based on past records.

The significant negative North Atlantic Oscillation (NAO) index in 2009-2010 also contributed to this extreme SLR event.

Climate models show that future changes in the overturning circulation are likely to increase both magnitude and frequency of extreme interannual sea level rise events along the Northeast coast of North America over the 21st century.

You can read more about the 2009-2010 extreme sealevel rise event at the GFDL: An Extreme Event of Sea-level Rise along the Northeast Coast of North America in 2009–2010. The study abstract says in full:
The coastal sea levels along the Northeast Coast of North America show significant year-to-year fluctuations in a general upward trend. The analysis of long-term tide gauge records identified an extreme sea-level rise (SLR) event during 2009–10. Within this 2-year period, the coastal sea level north of New York City jumped by 128 mm. This magnitude of interannual SLR is unprecedented (a 1-in-850 year event) during the entire history of the tide gauge records. Here we show that this extreme SLR event is a combined effect of two factors: an observed 30% downturn of the Atlantic meridional overturning circulation during 2009–10, and a significant negative North Atlantic Oscillation index. The extreme nature of the 2009–10 SLR event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the twenty-first century, climate models project an increase in magnitude and frequency of extreme interannual SLR events along this densely populated coast.

I highlighted this in February 2013 in an article on Sea Level rising 3 to 4 times faster on US Atlantic coast.

Sorry New York, sorry Boston. This is not good news. Any extra-tropical storms like ex-hurricane Sandy will bring a storm surge on top of this extreme sea level rise.

Watch climate scientist Paul Beckwith from the University of Ottawa on Youtube:




Weakening of Atlantic ocean carbon sink


As the Atlantic Meridional Overturning Circulation slows, uptake of carbon dioxide also reduces. This means more carbon dioxide accumulates in the atmosphere increasing the greenhouse effect. The oceans do us a big favour by absorbing about 26 per cent of atmospheric CO2. While this increases ocean acidification which causes other problems for the marine food web, it has reduced the rate of global warming from the greenhouse effect.

A study by Pérez et al (2013) - Atlantic Ocean CO2 uptake reduced by weakening of the meridional overturning circulation - highlighted that the slowdown of the meridional overturning circulation was largely responsible for the reduction in carbon uptake. Here is the abstract in full:

Uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean declined rapidly between 1990 and 2006. This reduction in carbon dioxide uptake was related to warming at the sea surface, which—according to model simulations—coincided with a reduction in the Atlantic meridional overturning circulation. The extent to which the slowdown of this circulation system—which transports warm surface waters to the northern high latitudes, and cool deep waters south—contributed to the reduction in carbon uptake has remained uncertain. Here, we use data on the oceanic transport of volume, heat and carbon dioxide to track carbon dioxide uptake in the subtropical and subpolar regions of the North Atlantic Ocean over the past two decades. We separate anthropogenic carbon from natural carbon by assuming that the latter corresponds to a pre-industrial atmosphere, whereas the remaining is anthropogenic. We find that the uptake of anthropogenic carbon dioxide—released by human activities—occurred almost exclusively in the subtropical gyre. In contrast, natural carbon dioxide uptake—which results from natural Earth system processes—dominated in the subpolar gyre. We attribute the weakening of contemporary carbon dioxide uptake in the subpolar North Atlantic to a reduction in the natural component. We show that the slowdown of the meridional overturning circulation was largely responsible for the reduction in carbon uptake, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic CO2 storage in subpolar waters.

Critical slowing down of AMOC provides warning signal of possible phase shift


The scientists warn that if the circulation slows down too much, it could pass a tipping point and break down completely. The IPCC assessed the liklihood of this happening as at most a one in ten chance within this century. But the science and risk assessment still need a lot more research.

Rahmstorf modeled the AMOC anomaly and index out to 2100, showing that the downward trend is likely to continue for the rest of the century.


According to a study by Boulton et al (2014), the Atlantic Meridional Overturning Circulation (AMOC) exhibits two stable states in climate models. Shifts between these states are thought to have been important for past abrupt climate changes. Models have determined that critical slowing down of the AMOC subject to freshwater hosing (as presently taking place) provides up to 250 years early warning signal for AMOC collapse.

A study by Kuhlbrodt et al (2009) - An Integrated Assessment of changes in the thermohaline circulation - detailed and assessed the risks of a shutdown of the thermohaline circulation (THC) for the climate system, as well as for ecosystems in and around the North Atlantic including for fisheries and agriculture. The abstract advises:
The climate model simulations are based on greenhouse gas scenarios for the 21st century and beyond. A shutdown of the THC, complete by 2150, is triggered if increased freshwater input from inland ice melt or enhanced runoff is assumed. The shutdown retards the greenhouse gas-induced atmospheric warming trend in the Northern Hemisphere, but does not lead to a persistent net cooling. Due to the simulated THC shutdown the sea level at the North Atlantic shores rises by up to 80 cm by 2150, in addition to the global sea level rise. This could potentially be a serious impact that requires expensive coastal protection measures.

This study advises that changes in currents in Nordic seas may substantially reduce the economic sustainability of the cod fishery this century. But on the positive side, crop productivity may be enhanced due to increased fertilization from atmospheric CO2 concentration and the increasing level of warming in northern Europe.

The paper argues that limiting the risk of a THC breakdown to 10% or less are narrow, requiring departure from business-as-usual in the next few decades of CO2 emissions. "The uncertainty about THC risks is still high." concludes the study.

It comes down to we need to cap global emissions to avoid destabilising the Atlantic Meridional Overturning Circulation past a tipping point where it collapses into it's other stable state. Another argument for strong action at the UN climate conference and proposed agreement in Paris in December 2015.

The shutdown of the North Atlantic Current, the Atlantic Meridional Overturning Circulation occurred last about 12,000 years ago, in the Younger Dryas, when a huge amount of freshwater flooded into the North Atlantic. Michael Mann explains the dynamics in an interview with Peter Sinclair and why we are unlikely to see anything as dramatic this time round with the present slowing of the AMOC.

Here is Peter Sinclair's compilation Youtube video of what scientists are presently saying about the Atlantic Meridional Overturning Circulation, and some clips from The Day after Tomorrow movie and scientists comments on the movie and it's use of creative licence to exaggerate and dramatise the science as featured in the movie.




Updated: 19:15 27 March 2015 - Added extra images, some text.

Sources:
  • Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change (online) [DOI:10.1038/nclimate2554] (abstract)
  • Potsdam Institute for Climate Impact Research media release 24 March 2015 - Atlantic Ocean overturning found to slow down already today
  • Goddard, P.B., Jianjun Yin, Stephen M. Griffies and Shaoqing Zhang. (2015) An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010 Nature Communications. DOI: 10.1038/ncomms7346. (abstract)
  • Boulton, C.A., Lesley C. Allison and Timothy M. Lenton (2014) Early warning signals of Atlantic Meridional Overturning Circulation collapse in a fully coupled climate model. Nature Communications 5, Article number: 5752 doi:10.1038/ncomms6752 Full article)
  • Pérez, F.F., Herlé Mercier, Marcos Vázquez-Rodríguez, Pascale Lherminier, Anton Velo, Paula C. Pardo, Gabriel Rosón and Aida F. Ríos (2013) Atlantic Ocean CO2 uptake reduced by weakening of the meridional overturning circulation. Nature Geoscience 6, 146–152 (2013) doi:10.1038/ngeo1680 (abstract)
  • Kuhlbrodt, T., Stefan Rahmstorf, Kirsten Zickfeld, Frode Bendiksen Vikebø, Svein Sundby, Matthias Hofmann, Peter Michael Link, Alberte Bondeau, Wolfgang Cramer, Carlo Jaeger (2009) An Integrated Assessment of changes in the thermohaline circulation. Climatic Change October 2009, Volume 96, Issue 4, pp 489-537 (abstract)
  • National Oceanography Centre media release 23 March 2015 - Ten years of ocean monitoring uncovers secrets of changing UK winters